Wall clocks and the like are available commercially which provide for remote resetting or remote synchronization, in order that the time displayed at a particular location will be accurately synchronized with time signal information broadcast from a central control point. A number of systems utilized for this purpose involve the reception and processing of time signal information which is constantly broadcast from government operated radio stations, such as WWVA and WWVB. Clock systems of this type, while highly accurate, are complex, usually quite expensive, and generally not suitable for household use.
A more economical form of resettable clock system involves the use of mechanical clutches or the like allowing the hands of the clock to be reset to a desired reference time in response to an external signal. These systems, while more economical than the systems described above, are somewhat lacking in accuracy, due to the involvement of mechanical clutch devices, and are nevertheless sufficiently costly as to limit their utilization principally to industrial and commercial activities.
In accordance with the present invention, a novel and improved clock systems is provided, which enables a highly accurate wall clock or the like to be provided and controlled at extremely low cost, sufficiently low to be suitable and attractive for general household usage, as well as commercial and industrial usage.
The clock system of the invention employs a unique and highly simplified control system for driving and controlling the hands or other timekeeping elements of the clock and accommodating resetting of the timekeeping elements to a precisely controlled reference time on a once-a-day basis.
The system of the invention advantageously employs a quartz crystal driven pulse generator, which can operate from conventional dry cell batteries and which is arranged to deliver a pulse output corresponding to one pulse per second. The output pulses serve to drive the three hands of the clock, which are geared together for movement in fixed ratios corresponding to seconds, minutes and hours. A simple stepper motor or the like may be used to advantage for driving the gear system for the clock hands. A series of cascaded counters are provided for maintaining a precise count of the output pulses. Resetting of the clock is performed on a daily basis, in response to a accurately timed signal received from a local radio station. The procedures followed for effecting the daily resetting are simple, and may be accomplished with inexpensive components, yet provide for exceptionally reliable and accurate time keeping.
Pursuant to one advantageous form of the invention, a timing signal is received at a precisely controlled time each day, preferably exactly at 02:00:00 am, when any resetting of the clock may be accomplished with minimum inconvenience. A few moments before the 02 am reset time, for example, at 01:59:50 am as indicated by the clock's internal counting system, the rate of pulses delivered to the clock drive is increased, preferably to two pulses per second. Accordingly, the clock hands reach the exact 02:00 am position approximately five seconds ahead of the time indicated by the clock's counter system. When the clock hands are in the exact 02:00 am position, further pulse input to the clock driving system is automatically interrupted and the clock hands remain fixed. At the exact moment of 02:00 am, as determined by a precision master clock at the central broadcasting station, a time signal is broadcast and is detected by the remote clock system. This time signal immediately resumes the flow of actuating pulses to the clock drive system and, at the same time, resets the clock counter to zero.
Although advancing the clock hands five seconds prior to the 02:00 am reset time is typically more than adequate to accommodate any variation in a quartz driven time keeping system (normal accuracy within one or two seconds per day), there may be circumstances in which the reset signal is not received from the broadcast station within a reasonable time window, e.g., ten seconds. In such cases, the clock system is restarted without adjustment, except to restore the few seconds during which the clock drive was held motionless as a result of interruption of pulses to the drive system. Additional provisions are made so that, if a number of successive days pass without the proper reception of a reset signal, the clock will be stopped at the 02:00 A.M. reset position and not restarted, as an indication to the user of a malfunction in the reset system.
In another advantageous form of the invention, the receipt of the reset signal (provided it is detected within a brief, ten second window provided) triggers a system which temporarily stores the reading of the "seconds" counter, in a range from zero to nine (four line BCD output), at the moment of the instant of the reset signal. A "seconds" counter reading in the range of one to four, indicates that the clock is one to four seconds fast, and a corresponding number of pulses is subtracted from the regular flow of such pulses, so that both the counter and the timekeeping mechanism are resynchronized with the time reference provided by the reset signal. If the counter value at the moment of the reset signal is in the range of six to nine, that indicates that the clock is four to one seconds slow, and an appropriate number of extra pulses is added to the pulse stream to synchronize the clock with the reference signal.
In any of its various forms, the clock system of the invention includes a start-up feature which functions during the initial start-up of the clock, after battery changes, or when a malfunction in the broadcast/receiving system prevents the clock from detecting the reference signal for a predetermined number of days (typically ten days). The start-up condition of the clock is indicated by a "flag" address in the logic chip controller which initially reflects an "unset" condition of the clock. When the clock is powered up and started for the first time, the unset flag causes the pulse-actuated clock drive to receive pulses at a greatly increased rate relative to normal, for example, ten pulses per second. The clock is thus driven in a relatively short period of time to the position in which it can expect to receive a reset signal, for example, 02:00:00 am. The clock hands automatically are stopped in that position by the interruption of the pulse stream and the system awaits the arrival of the next reset signal. When the signal is received, the clock commences normal operations, and the start-up flag is set to indicate normal operations.
To particular advantage, the system of the invention provides for automatic adjustment of the clock to accommodate the beginning and ending of daylight saving time. On the respective days for the beginning and ending of daylight saving time, the time keeping signal sent by the broadcasting station is of a distinctive character recognizable by the tuned receiver of the clock system and which operates to add or subtract exactly one hour from the pulse train to the clock driving system.
It is contemplated that the clock system of the invention will be driven by a quartz crystal oscillator type pulse generating system. Such systems are widely used and very inexpensive, and have a level of accuracy reliably within one or two seconds per day. Accordingly, by providing for daily reset of the clock, it is assured that timekeeping accuracy will be extremely high by normal household and commercial standards, suitable for all but the most demanding applications. At the same time, the system can be made available at a cost only modestly above that of conventional. household clocks provided with manual reset features. Among other things, the clock of the invention eliminates all manual reset facility, so that the additional cost of the electronic control system is offset in significant part by the elimination of mechanical parts otherwise required.
The system of the invention may take a variety of specific forms each, however, being characterized by a high level of simplicity and economy, so that a simple, low cost, battery driven clock may be provided with an accurate daily reset feature, automatic start up, and automatic adjustment for daylight saving time.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of several preferred embodiments of the invention, and to the accompanying drawings.